Seals or packoffs are typically used as a pressure barrier in the annular space between inner and outer wellhead tubular members for containing internal well pressure. The inner wellhead member may be a casing hanger located in a wellhead housing and that supports a string of casing extending into the well or a tubing hanger that supports a string of tubing extending into the well for the flow of production fluid. Casing hangers generally are landed in a wellhead housing whereas tubing hangers are typically landed in one of a wellhead housing, a Christmas tree, or a casing hanger.
A variety of seals of this nature have been employed in the prior art. Prior art seals include elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely of metal for forming metal-to-metal seals are also employed. The seals may be set by a running tool, or they may be set in response to the weight of the string of casing or tubing. One type of prior art metal-to-metal seal is U-shaped, having inner and outer walls separated by a cylindrical slot. A wedge-shaped energizing ring is pushed into the slot in the seal to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members. The deformation of the seal's inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent.
The U-shaped geometry of the seal allows bore pressure to act on the legs and thereby improve sealing with increased pressure. However, pressure in the annulus below the casing hanger has the opposite effect on the seal and will result in a leak if the pressure is great enough. Further, the bore pressure tends to degrade the performance of the annulus seal over time. This is because the contact pressure at the sealing surfaces of the seal is not only enhanced by the U-shaped geometry but also the hanger neck geometry, which further compresses the sealing surfaces when the hanger is exposed to pressure along its bore. These two pressure enhancing factors typically exceed the preload of the annulus seal, resulting in plastic deformation that may decrease contact force in the sealing surfaces over time and in turn cause leaks.
One approach taken to address this leakage problem in metal-to-metal seals has been the addition of a set of wickers to the exterior of the casing hanger and the bore of the wellhead housing. The wickers on both the casing hanger and wellhead housing sealingly engage the sealing surfaces of the U-seal after they are deformed by the energizing ring. The wickers aim to prevent axial movement of the seal and focus the radial sealing force over a narrow band. However, with increases in production pressure, pressure cycles, and plastic deformation of the seal's contact surfaces, leaks may still develop in the seal.
A need exists for a technique that addresses the seal leakage problems described above. In particular a need exists for a technique to maintain a seal between inner and outer wellhead members experiencing changes in relative positions due to thermal affects, especially those caused by high pressure and pressure cycle wellbore conditions. The following technique may solve these problems.